I have just discovered that if we configure the Jenkins job to contain
wildcards in the Branch Specifier field, e.g. origin/personal/**, then the
polling of the job would just hang after the first line:

Git Polling Log

Started on Oct 24, 2014 4:37:58 PM

This would stay like that until the currently running build of this job
completes. Then polling would go through in sub-seconds. I tried specifying a
fully spelt-out branch specifier, and this problem goes away.

Normally this would not be a problem, but we are now trying to enable
concurrent builds for this job. That means, as long as there's one build in
progress for this job, no additional builds can start (since polling is
hanging) until the current build finishes. So this bug effectively breaks
concurrent jobs that relies on polling.